Why Compass Accuracy Matters More Than You Think
In the age of GPS chartplotters, it's tempting to treat the magnetic compass as a backup curiosity. But experienced mariners know better: electronics fail, batteries die, and lightning strikes happen. A properly corrected compass — and the knowledge to use it — is your ultimate navigation safety net.
More practically, even if you never navigate by compass alone, understanding deviation and variation helps you interpret your compass correctly, set up your autopilot accurately, and make sense of the difference between what your compass says and what your chartplotter says.
This article explains the two sources of compass error — variation and deviation — clearly and practically.
True North vs. Magnetic North
Before diving into deviation and variation, it helps to understand the three "norths" used in navigation:
- True North: The geographic North Pole — the axis around which the Earth rotates. Charts are drawn with True North at the top.
- Magnetic North: The location of the Earth's magnetic north pole, which is not the same as True North and moves slowly over time. A compass needle points to Magnetic North.
- Compass North: What your specific compass actually reads, which may differ from Magnetic North due to local magnetic influences on your boat.
The difference between True North and Magnetic North is called variation. The difference between Magnetic North and Compass North is called deviation. Together, they account for the total error between what your compass reads and the true direction you're heading.
Magnetic Variation
What It Is
Magnetic variation (also called magnetic declination) is the angular difference between True North and Magnetic North at any given location on Earth. It's caused by the fact that the Earth's magnetic pole is not located at the geographic pole — and it moves over time as the magnetic pole drifts.
Variation is a property of your location, not your boat. It's the same for every compass at the same location.
How Much Does It Matter?
Variation varies significantly by location:
- Along the US East Coast: approximately 10–15° West (your compass reads 10–15° too high)
- Along the US Gulf Coast: approximately 1–5° East to West depending on location
- Along the US West Coast: approximately 12–18° East (your compass reads 12–18° too low)
- In some parts of the world, variation exceeds 20°
At 10° of variation, a boat steering a compass course of 090° (due East by compass) is actually heading 080° True — 10° off course. Over a 10-mile run, that's nearly 1.75 miles of lateral error.
Where to Find Variation
- Nautical charts: Variation is printed on the compass rose on every nautical chart, along with the annual rate of change. Always use the most current chart and apply the annual correction if the chart is several years old.
- Chartplotters: Most GPS chartplotters automatically apply variation to display magnetic headings correctly. Check your chartplotter's settings to confirm whether it's displaying True or Magnetic headings.
- NOAA: The National Oceanic and Atmospheric Administration publishes current magnetic declination data for any US location at ngdc.noaa.gov.
Correcting for Variation
The classic memory aid: "Variation East, Magnetic Least; Variation West, Magnetic Best."
- If variation is East: Magnetic heading is less than True heading. To convert True to Magnetic, subtract easterly variation.
- If variation is West: Magnetic heading is greater than True heading. To convert True to Magnetic, add westerly variation.
Example: You want to steer True 045°. Variation is 12° West. Magnetic course = 045° + 12° = 057° Magnetic. Steer 057° on your compass (before applying deviation).
Compass Deviation
What It Is
Deviation is the error in your specific compass caused by magnetic influences on your boat — the engine, electrical wiring, speakers, electronics, metal fittings, and even the steel in your hull. Unlike variation, deviation is unique to your boat and changes depending on which direction you're heading.
This is why deviation is expressed as a deviation table — a list of compass headings and the corresponding deviation on each heading. A compass might have +3° deviation on a North heading, -5° on an East heading, and +1° on a South heading.
Why Deviation Changes With Heading
The magnetic fields on your boat are fixed relative to the boat. As the boat turns, the relationship between those fixed magnetic fields and the Earth's magnetic field changes — so the error they introduce into the compass reading changes with heading.
Sources of Deviation on a Boat
- The engine block and other large iron/steel components
- DC electrical wiring carrying current (current creates magnetic fields)
- Speakers (contain powerful magnets)
- Electronics — VHF radios, fishfinders, and other devices
- Steel fuel tanks
- Anchors and chain stored near the compass
- Tools left near the compass
Minimizing Deviation at Installation
The best time to minimize deviation is when installing the compass. Follow these guidelines (covered in more detail in our compass installation guide):
- Mount the compass at least 18–24 inches from speakers, radios, and large metal objects
- Avoid mounting directly above or below electronics
- Run DC wiring away from the compass, and use twisted-pair wire where possible to cancel magnetic fields
- Use the compass's built-in compensating magnets to reduce deviation on the N-S and E-W axes
Creating a Deviation Table
After installation and compensation, you need to measure the remaining deviation on multiple headings and record it in a deviation table. Here's how:
- Choose a calm day with light winds and minimal current
- Go to open water away from other vessels, structures, and shallow areas
- Use a known reference: Your GPS chartplotter displays your actual magnetic heading (Course Over Ground, or COG). Compare this to your compass reading on each heading.
- Steer each of 8 headings (N, NE, E, SE, S, SW, W, NW) and hold each heading steadily for 30–60 seconds
- Record the difference between your GPS COG and your compass reading on each heading. If the compass reads higher than the GPS, deviation is West (negative). If the compass reads lower, deviation is East (positive).
- Create your table and post it near the compass
Example deviation table:
| Compass Heading | Deviation | Magnetic Heading |
|---|---|---|
| 000° (N) | +3° E | 003° |
| 045° (NE) | +1° E | 046° |
| 090° (E) | -2° W | 088° |
| 135° (SE) | -4° W | 131° |
| 180° (S) | -3° W | 177° |
| 225° (SW) | -1° W | 224° |
| 270° (W) | +2° E | 272° |
| 315° (NW) | +3° E | 318° |
Correcting for Deviation
The same memory aid applies: "Deviation East, Compass Least; Deviation West, Compass Best."
- If deviation is East: Compass reads less than Magnetic. To get Magnetic from Compass, add easterly deviation.
- If deviation is West: Compass reads more than Magnetic. To get Magnetic from Compass, subtract westerly deviation.
Putting It All Together: True, Magnetic, and Compass
The full correction chain, from True course (on the chart) to Compass course (what you steer):
True → apply Variation → Magnetic → apply Deviation → Compass
And the memory aid for the full chain: "True Virgins Make Dull Company" (True, Variation, Magnetic, Deviation, Compass) — add West errors, subtract East errors when converting from True to Compass.
Full example:
- True course from chart: 060°
- Variation: 12° West → add 12° → Magnetic course: 072°
- Deviation on ~072° heading: 2° West → add 2° → Compass course: 074°
- Steer 074° on your compass to make good a True course of 060°
When to Re-Swing Your Compass
Deviation changes whenever the magnetic environment on your boat changes. Re-create your deviation table after:
- Installing new electronics, speakers, or electrical equipment near the compass
- Adding or moving metal equipment (anchors, tools, tanks)
- Significant engine or electrical system work
- Any time you notice the compass and GPS disagree by more than expected
- At minimum, every 2–3 years as a routine check
Shop Marine Compasses at Egret Marine
Browse our full selection of Marine Navigation & Instruments, including the complete Ritchie compass lineup — the SS-1002, FN-201, F-50, BN-202, and more. Every Ritchie compass includes built-in compensating magnets for deviation correction.
Related Articles
- How to Install a Marine Compass: Flush Mount vs. Bulkhead Mount
- How to Choose a Fishfinder/Chartplotter Combo for Your Boat
- How to Set Up an NMEA 2000 Network on Your Boat
- Anchoring 101: How to Choose the Right Anchor and Rode for Your Boat
Questions about compass selection or installation for your boat? Contact the Egret Marine team — we're happy to help.